Electronic noise reduction systems have the general object of removing unwanted noise from signals containing useful information. Simple noise reduction systems commonly use filters to attenuate noise expected to reside primarily in certain definable frequency bands. The successful use of such a system depends in large measure on how different the frequency components of the signal and noise are. Very often, the signal and noise will have overlapping spectra, and such a noise reduction system will tend to alter the spectral characteristics of the desired signal. In more sophisticated noise reduction systems, the general object has been to increase the signal-to-noise ratio without significantly distorting the characteristics of the desired signal.
In the reproduction of music, high-frequency noise such as tape hiss has been a problem. A system has been proposed for reducing such high-frequency noise without pre-processing source signals and without unduly impairing the effective frequency response of the associated sound reproduction system. In the proposed noise reduction system, the desired signal is processed by a dynamic low-pass filter whose corner or cut-off frequency can be varied during operation. To ensure high fidelity sound reproduction, the cut-off frequency is varied in response to the detected instensity of high-frequency signal components, the pass-band of the filter being expanded to accommodate the frequency content of the incoming signal and otherwise restricted to reduce reproduction of high-frequency noise. Such a system is described in an article entitled "On-Chip Stereo Filter Cuts Noise Without Pre-processing Signals" by Martin Giles, in Electronics, Aug. 11, 1981 P.104.
An analogous approach has been proposed in hearing aid applications in which low-frequency noise is a common problem. In the proposed system, a signal channel containing a dynamic high-pass filter with a variable cut-off frequency is provided. A control path is provided which includes a low-pass filter, rectifier and smoothing filter, which produce a DC voltage indicative of the intensity or strength of the low-frequency signal components, ideally noise components. This voltage is applied to the high-pass filter in the signal channel to control filter bandwidth. When low-frequency noise is detected, the pass-band of the filter is restricted, ideally attenuating the noise. Such a system is described in an article entitled "Automatic Noise Suppression in Hearing Aids", by S. Iwasaki, in the Hearing Aid Journal, December 1981, P.10. There are two basic problems with such a system. First, low-level speech signals may be obscured by noise at a low-level insufficient to cause restriction of the bandwidth of the high-pass filter. Second, to prevent restriction of filter band width in response to low-frequency components of the male voice, the low-pass filter in the control channel must be restricted to a pass only frequencies below 100 Hz. With such a requirement imposed on the low-pass filter, the system works well only with very low-frequency noise. The alternative is to allow normal operation of the highpass filter with a more restricted band width, resulting in reproduced speech with a "tinniness" which is unpleasant to the human ear and which lacks most of the distinctive qualities of the speaker's voice. More elaborate noise reduction systems have been proposed which involve a multiplicity of computer-control filters in the signal channel. The associated computer analyzes the frequency of the temporal characterisitics of the desired signal to determine if noise is present. When noise is detected, the associated computer adjusts the filters using an iterative technique until the noise of the output of the filters is minimized. Such a system works extremely well if the algorithms are sophisticated enough to reliably differentiate between speech and noise and if a sufficient number of filters is provided. The major disadvantages of such an approach are size, cost, complexity and power consumption.
It is an object of the present invention to provide method and apparatus for enhancing the intelligibility of speech signals containing low-frequency noise devoid of many of the drawbacks associated with prior art noise reduction systems. Various objects and advantages of the invention will be described in greater detail below.